Although most young children seem to master reading skills in the early grades of elementary school, many struggle with texts as they move through middle school and high school. Why do children who seem to be proficient readers in third grade have trouble comprehending texts in later grades? To answer this question, Nonie Lesaux describes what is known about reading development and instruction, homing in on research conducted with children from low-income and non-English-speaking homes. Using key insights from this research base, she offers two explanations. The first is that reading is a dynamic and multifaceted process that requires continued development if students are to keep pace with the increasing demands of school texts and tasks. The second lies in the role of reading assessment and instruction in U.S. schools.

Lesaux draws a distinction between the "skills-based competencies" that readers need to sound out and recognize words and the "knowledge-based competencies" that include the conceptual and vocabulary knowledge necessary to comprehend a text's meaning. Although U.S. schools have made considerable progress in teaching skills-based reading competencies that are the focus of the early grades, most have made much less progress in teaching the knowledge-based competencies students need to support reading comprehension in middle and high school. These knowledge-based competencies are key sources of lasting individual differences in reading outcomes, particularly among children growing up in low-income and non-English-speaking households.

Augmenting literacy rates, Lesaux explains, will require considerable shifts in the way reading is assessed and taught in elementary and secondary schools. First, schools must conduct comprehensive reading assessments that discern learners' (potential) sources of reading difficulties—in both skills-based and knowledge-based competencies. Second, educators must implement instructional approaches that offer promise for teaching the conceptual and knowledge-based reading competencies that are critical for academic success, particularly for academically vulnerable populations.